Various metal oxide films have been suggested as dielectric or ferroelectric layers in the microelectronics industry. For instance, lead zirconate titanate, barium titanate, strontium bismuth tantalate, strontium bismuth niobate, and barium strontium titanate as well as other metal oxides have been suggested as the dielectric or ferroelectric capacitors used in memory cells. Metal oxide films such as BaSrTiO.sub.3 (BST) films have many potential applications in dynamic random access memories (DRAM), nonvolatile random access memories (NVRAM), field effect transistor (FET) display technology and passive component packaging (interposer). Successful solution deposition of ferroelectrics or dielectrics such as BaSrTiO.sub.3 (BST) depends on the availability of suitable precursors with appropriate reactivity, low cost and nontoxicity.
For example, Sol-gel deposition of BST and other dielectric metal oxide materials is typically conducted with short-chain alkoxides such as ethoxides, or isopropoxides. Short-chain alkoxides usually hydrolyze too readily to permit convenient handling and long term storage. These alkoxides will absorb ambient water from the atmosphere, which quickly hydrolyzes some of the material and can lead to precipitate formation.
Other liquid deposition routes to BST include metal-organic decomposition routes based on thermal decomposition of metal acetates. Acetate based chemistry typically requires higher annealing temperatures and passes through an oxocarbonate transition to form barium strontium titanate. In addition, acetate solutions are typically unstable and can gel within hours or days of mixing.
An improvement on short-chain alkoxides or MOD with acetates is sol-gel deposition with barium strontium and titanium methoxyethoxides. Barium titanium methoxyethoxides are known (J. -F. Campion, D. A. Payne, H. K. Chae, J. K. Maurin, S. R. Wilson, Inorg. Chem. Volume 30, pg. 3245, 1991; K. G. Caulton, M. H. Chisolm, S. R. Drake, J. C. Huffman, J. Chem. Soc. Chem. Comm., pg. 1498 (1990) and have been fabricated into thin films (J. -F. Campion, D. A. Payne, H. K. Chae and Z. Xu, Ceramic Trans. Volume 22, pg. 477, 1991; M. H. Frey, D. A. Payne, Appl. Phys. Lett. Volume 63, pp. 2753, 1993). Strontium titanium 2-methoxyethoxides are known and have been fabricated into thin films (U. Selvarag, A. V. Prasadarao, S. Komarneni, R. Roy, Materials Letters Volume 23, pg. 123, 1995).
Methoxyethoxides hydrolyze more slowly than acetates or short chain alkolides, resulting in improved moisture tolerance and solvent stability. However, barium strontium titanium methoxyethoxide mixtures have limited solubility, limiting solution concentrations to less than 0.4 M. This limited solubility necessitates storage of dilute stock solutions, difficulty in spin coating or dipping thick layers, and solubility problems during extended storage, and precipitation during spin coating or dipping.
Barium titanium methoxyethoxides have been shown to form partially hydrolyzed clusters after exposure to trace amounts of water which leads to a degradation of the spin solution and results in films with poorer electrical properties. An additional problem associated with methoxyethanol is its tetragenic toxicity. Methoxyethanol is banned in many industrial applications and is listed as a hazardous waste product, limiting its usefulness for commercial applications.